transaction: allow to backup file that already have an offset

This will be useful in the next changeset to deal with rolling back the split of
inline revlog on transaction failure.

This involve deeper change to the transaction logic as we need to make sure we
restore the backup -before- the truncation step. Otherwise, the truncation
would act on the wrong file and be overwritten by the backup restoration later.

mercurial/transaction.py

@@ -105,6 +105,11 b' def _playback('
     unlink=True,
     checkambigfiles=None,
 ):
+    """rollback a transaction :
+    - truncate files that have been appended to
+    - restore file backups
+    - delete temporary files
+    """
     backupfiles = []
 
     def restore_one_backup(vfs, f, b, checkambig):
@@ -118,7 +123,30 b' def _playback('
             report(_(b"failed to recover %s (%s)\n") % (f, e_msg))
             raise
 
+    # gather all backup files that impact the store
+    # (we need this to detect files that are both backed up and truncated)
+    store_backup = {}
+    for entry in backupentries:
+        location, file_path, backup_path, cache = entry
+        vfs = vfsmap[location]
+        is_store = vfs.join(b'') == opener.join(b'')
+        if is_store and file_path and backup_path:
+            store_backup[file_path] = entry
+    copy_done = set()
+
+    # truncate all file `f` to offset `o`
     for f, o in sorted(dict(entries).items()):
+        # if we have a backup for `f`, we should restore it first and truncate
+        # the restored file
+        bck_entry = store_backup.get(f)
+        if bck_entry is not None:
+            location, file_path, backup_path, cache = bck_entry
+            checkambig = False
+            if checkambigfiles:
+                checkambig = (file_path, location) in checkambigfiles
+            restore_one_backup(opener, file_path, backup_path, checkambig)
+            copy_done.add(bck_entry)
+        # truncate the file to its pre-transaction size
         if o or not unlink:
             checkambig = checkambigfiles and (f, b'') in checkambigfiles
             try:
@@ -137,12 +165,16 b' def _playback('
                 report(_(b"failed to truncate %s\n") % f)
                 raise
         else:
+            # delete empty file
             try:
                 opener.unlink(f)
             except FileNotFoundError:
                 pass
-
-    for l, f, b, c in backupentries:
+    # restore backed up files and clean up temporary files
+    for entry in backupentries:
+        if entry in copy_done:
+            continue
+        l, f, b, c = entry
         if l not in vfsmap and c:
             report(b"couldn't handle %s: unknown cache location %s\n" % (b, l))
         vfs = vfsmap[l]
@@ -170,6 +202,7 b' def _playback('
             if not c:
                 raise
 
+    # cleanup transaction state file and the backups file
     backuppath = b"%s.backupfiles" % journal
     if opener.exists(backuppath):
         opener.unlink(backuppath)
@@ -346,7 +379,7 b' class transaction(util.transactional):'
         self._file.flush()
 
     @active
-    def addbackup(self, file, hardlink=True, location=b''):
+    def addbackup(self, file, hardlink=True, location=b'', for_offset=False):
         """Adds a backup of the file to the transaction
 
         Calling addbackup() creates a hardlink backup of the specified file
@@ -355,17 +388,25 b' class transaction(util.transactional):'
 
         * `file`: the file path, relative to .hg/store
         * `hardlink`: use a hardlink to quickly create the backup
+
+        If `for_offset` is set, we expect a offset for this file to have been previously recorded
         """
         if self._queue:
             msg = b'cannot use transaction.addbackup inside "group"'
             raise error.ProgrammingError(msg)
 
-        if (
-            file in self._newfiles
-            or file in self._offsetmap
-            or file in self._backupmap
-        ):
+        if file in self._newfiles or file in self._backupmap:
+            return
+        elif file in self._offsetmap and not for_offset:
             return
+        elif for_offset and file not in self._offsetmap:
+            msg = (
+                'calling `addbackup` with `for_offmap=True`, '
+                'but no offset recorded: [%r] %r'
+            )
+            msg %= (location, file)
+            raise error.ProgrammingError(msg)
+
         vfs = self._vfsmap[location]
         dirname, filename = vfs.split(file)
         backupfilename = b"%s.backup.%s" % (self._journal, filename)